Every interactive object should have a significant impact on game state.

In other words, there should be no frivolous interactions.

Counter-examples

Examples of interactive objects that appear in many games and do not affect gameplay:

1. Turning on/off lights where lighting has no gameplay impact.
2. Flushing toilets.

Duke Nukem 3D

Duke Nukem 3D is notable for both the density of interactive actors it contains, and for the quantity of them that do not affect gameplay.

That worked for that game and was a good selling point, but I want to have a steady composition of both style and substance, not just style.